Multi-processor computing systems are becoming increasingly more common in a variety of applications. A multi-processor system is one which includes multiple processors, where the processors can be physical processors, logical processors, or a combination thereof. A single physical processor can implement multiple logical processors, as illustrated in FIG. 1, in which one physical processor 6 includes two logical processors 7. In such an implementation, the logical processors generally have some private state, but a portion of the state is shared. Henceforth in this document, the term “processor” is intended to mean either a physical processor or a logical processor unless the term is otherwise qualified.
To ensure that instructions and data are safe for execution in a multi-processor environment, the various processes implemented by the operating system can be organized into a number of mutual exclusion domains according to their functionality. A “domain”, in this context, is a grouping of processes. Every process capable of execution by the processing device is assigned to exactly one domain. The domains are defined according to functionality, so that it is not possible for two processes in different domains to operate on the same data simultaneously. Furthermore, only one process at a time can execute in each domain (with a few exceptions for operations that are inherently multi-processor safe). Further details of a technique for defining and using domains in this manner are described in co-pending U.S. patent application Ser. No. 09/828,271 of V. Rajan et al, filed on Apr. 5, 2001 and entitled, “Symmetric Multi-Processor Synchronization using Migrating Scheduling Domains (“Rajan”), and U.S. patent application Ser. No. 09/828,284 of C. Peak et al., filed on Apr. 5, 2001 and entitled, “Automatic Verification of Scheduling Domain Consistency” (“Peak”), both of which are incorporated herein by reference.
The above-mentioned technique can be implemented in a storage server, such as one of the various models of Filer made by Network Appliance, Inc. (NetApp®) of Sunnyvale, Calif. The domains can be organized according to the critical path pipeline of the storage server. For example, when a storage server receives a data access request (read or write) from a client over a network, a network software layer of the storage server sends an appropriate message to the storage server's file system, which processes the message to determine where the corresponding data is stored, and which then forwards a corresponding message to a storage software layer (e.g., RAID layer) of the storage server. Each of these phases of processing the request is carried out by a different stage in the pipeline; as such, a separate domain can be created for each stage, e.g., a domain for all network-specific processes of the storage server, a domain for all file system-related processes of the storage server, a domain for all storage-specific processes of the storage server, etc.
It has been observed in certain storage servers that the different pipeline stages (and, hence, the corresponding domains) tend to have different degrees of processor utilization. For example, the file system related domain tends to have much higher processor utilization (close to 100 percent in certain implementations) than the network and storage domains (typically in the range of 20 to 50 percent). The file system domain, therefore, tends to be a bottleneck in the critical path of the storage server, thus limiting the throughput of the storage server.